In recent years, miniaturization, weight reduction, and thickness reduction of electronic devices such as notebook computers and mobile phones have progressed. Accordingly, secondary storage batteries for electronic devices have been required to have improved performance, a lighter weight, and improved moving performance, and lithium ion batteries having a high energy density have been actively developed in place of lead storage batteries of the related art. Further, lithium ion batteries which can be used as power sources for electric vehicles or hybrid vehicles have come into practical use.
In a lithium ion battery, a compound containing lithium and a carbon material such as graphite or coke have been respectively used as a positive electrode material and a negative electrode material. Further, an electrolytic solution obtained by dissolving a lithium salt such as LiPF6 or LiBF4 as an electrolyte in an aprotic solvent having penetration capability such as propylene carbonate or ethylene carbonate or an electrolyte layer formed of a polymer gel impregnated with the electrolytic solution is provided between a positive electrode and a negative electrode.
In the related art, as a packaging material for a battery casing, a laminate obtained by sequentially laminating a heat-resistant resin stretched film layer and an aluminum foil layer serving as outer layers and a thermoplastic resin unstretched film layer serving as an inner layer has been known. In a battery case obtained using a packaging material for a battery casing having such a structure, in a case where a solvent having penetration capability such as an electrolytic solution passes through a film layer serving as a sealant in the packaging material for a battery casing, the laminate strength between the aluminum foil layer and the resin film layer is degraded and this may cause leakage of the electrolytic solution. Therefore, a packaging material for a battery casing obtained by bonding the aluminum foil layer and the inner layer through an adhesive layer that contains a polyfunctional isocyanate compound and a resin containing a functional group having reactivity with isocyanate such as an acid anhydride group, a carboxy group, or a hydroxy group has been developed.
For example, PTL 1 describes a method of forming an adhesive layer using a modified polyolefin resin obtained by grafting ethylenically unsaturated carboxylic acid or an anhydride thereof onto a homopolymer of propylene or a copolymer of propylene and ethylene and a solvent type adhesive obtained by dissolving or dispersing a polyfunctional isocyanate compound in an organic solvent.
In addition, PTL 2 describes an adhesive composition which contains polyolefin polyol and a polyfunctional isocyanate curing agent as indispensable components and to which a thermoplastic elastomer and/or a tackifier has been added. Further, PTL 3 describes an adhesive composition which contains one or more main agents selected from the group consisting of polyester polyol having a hydrophobic unit derived from a dimer fatty acid or a hydrogenated material thereof and an isocyanate elongation material of the polyester polyol; and a curing agent formed of one or more polyisocyanate compounds selected from the group consisting of crude tolylene diisocyanate, crude diphenylmethane diisocyanate, and polymeric diphenylmethane diisocyanate.